Single-walled carbon nanotubes (SWNTs) possess unique structural, electronic, mechanical, and optical properties (Dresselhaus et al. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications, Springer Verlag, Berlin, 2001; Falvo et al., Nature 1997, 389, 582). The combination of the helicity and diameter of SWNTs, defined by the roll-up vector, i.e. tube chirality, determines whether a tube is a metal or a semiconductor. One main advantage of understanding the electronic structure of carbon nanotubes is that as spatially constrained one-dimensional structures, they are the smallest dimensional systems that can be used for the efficient transport of electrons and optical excitations and hence are expected to be particularly important in the construction and integration of nanoscale devices.
The majority of electronics applications specifically require the isolation of semiconducting tubes (Bachtold et al., Science 2001, 294, 1317; Baughman et al., Science 2002, 297, 787; Wong et al., Nature 1998, 394, 52; Yang et al., J. Phys. Chem. B 2002, 106, 8994). However, the lack of control over the electronic properties of as-prepared nanotubes, e.g. the inability to reliably separate masses of semiconducting from metallic tubes, has created a major stumbling block for their incorporation into functional devices. Thus, there is an urgent need to obtain electronic monodispersity in nanotube samples. Generating such monodisperse samples of nanotubes should also allow for detailed studies of diameter and chirality dependence of many different structural properties that are of fundamental interest in low-dimensional science.
One solution to this problem involves the controllable use of covalent chemistry to modify the sidewall surfaces of tubes to enhance the relative populations of either metallic or semiconducting tubes. Some of these types of functionalization reactions, such as osmylation and diazotisation, involve extraction of electrons from the nanotube itself (Bahr et al., J. Am. Chem. Soc. 2001, 123, 6536; Banerjee et al., J. Am. Chem. Soc. 2004, 126, 2073; Dyke et al., Nano Lett. 2003, 3, 1215). In particular, with these reactions, metallic SWNTs, due to their finite and readily available electron density at the Fermi level, are better able to stabilize the transition state involved, will consequently accelerate the forward rate of reaction, and hence will preferentially react as compared with semiconducting tubes. Another means of altering the relative distribution of metallic vs. semiconducting carbon nanotubes involves the chemical derivatization of nanotubes with a high K dielectric coating material, such as Si-containing species.
Prior work on coating SWNTs with SiO2 and analogous derivatives has focused on a number of methods. An early study reported on the use of a promoter layer, 3-aminopropyltriethoxysilane, followed by a modified Stöber reaction to generate silica (Fu et al., Nano Lett. 2002, 2, 329). Peptides have been used to both suspend SWNTs as well as direct the precipitation of silica onto their surfaces (Pender et al., Polymer Preprints (American Chemical Society; Division of Polymer Chemistry) 2005, 46, 83). Treatment of a dispersion of SWNTs in an aqueous surfactant solution with an acidic solution of fumed silica resulted in silica-coated SWNTs (Whitsitt et al., J. Mater. Chem. 2005, 15, 4678), whereas a complementary method for generating silica-coated SWNTs was devised using a basic solution of aqueous sodium silicate (Colorado et al., Adv. Mater. 2005, 17, 1634). Finally, the same group coated SWNTs with fluorine-doped silica by liquid phase deposition using a silica/H2SiF6 solution in the presence of surfactant.
Theoretical studies (Lu et al., J. Phys. Chem. B 2003, 107, 8388; Chu et al., Chem. Phys. Lett. 2004, 394, 231) have postulated that the [2+1]cycloaddition of silylene on nanotube sidewalls is site-selective, occurring preferentially on the 1,2-pair site and favoring opened structures (Zhang et al., J. Molec. Struct. (Theochem) 2004, 681, 225; Lu et al., J. Molec. Struct. (Theochem) 2005, 725, 255). Organosilanes have been extensively used as coupling agents on hydroxylated surfaces for generating organic coatings, with the idea that the electrical properties of carbon nanotubes can be appropriately adjusted through rational chemical functionalization (Duchet et al., Langmuir 1997, 13, 2271). In previous work, the silylation of oxidized multi-walled carbon nanotubes has been performed with a variety of reagents including tert-butylchlorodimethylsilane and 1-(tert-butyldimethylsilyl)imidazole (Velasco-Santos et al., Nanotechnology 2002, 13, 495; Vast et al., Nanotechnology 2004, 15, 781).
However, prior art attempts at silylation of nanotubes have many disadvantages. For example, prior art attempts at silylation of nanotubes spatially limited silylation to defect sites and ends. Moreover, prior silylation techniques required harsh oxidative methods.